It is well known that piezoelectric properties as an energy conversion function are characteristic of crystals not having a symmetric center. In fact, inorganic materials such as quartz crystal, Rochelle salt or lead zirconate titanate-based ceramics, are well used. These inorganic materials, however, have poor flexibility and are difficult to mold or process. Thus, piezoelectric materials having a large surface area, shaped in a thin membrane, or having a complicated form are very difficult to produce from inorganic materials, although they have a high piezoelectric modulus.
It is also known that stretched films, for example, of certain polymeric materials, e.g., natural polymers such as celluloses or proteins, and synthetic polymers such as poly-.gamma.-methyl-L-glutamate, have piezoelectric properties. Moreover, it is known that electrets of other synthetic polymers, e.g., those produced by applying a high DC voltage to stretched films of, e.g., polyvinyl fluoride, polyvinylidene fluoride (PVDF), polyvinyl chloride, polyacrylonitrile (PAN), polycarbonate or nylon 11, while maintaining them at a high temperature and thereafter cooling the films have piezoelectric properties and of these electrets, PVDF containing an orientated Type I (Type .beta.) crystal exhibits a high piezoelectric effect. These organic piezoelectric materials, however, have poor longitudinal piezoelectric effect although they are superior in moldability and flexibility. A high longitudinal piezoelectric effect is one of the objects that the present invention is intended to attain. For example, in the case of PVDF electrets, which exhibits the highest piezoelectric modulus, the longitudinal piezoelectric effect (Kt) is as small as 0.18 although they have a relatively high transverse piezoelectric effect (d.sub.31) as high as 40 pC/N. Thus, the PVDF electrets are subject to serious limitations when they are used, particularly in applications where a high longitudinal piezoelectric effect is required, e.g., as transmitting and receiving elements of ultrasonic transducers.
The present invention is intended to overcome the above defects of the prior art inorganic or organic piezoelectric materials and provide organic piezoelectric materials which are superior in moldability and flexibility and exhibit a high longitudinal piezoelectric effect.
As a result of extensive studies on the mechanism of generation of piezoelectricity in PVDF electrets which are considered to have the highest piezoelectric modulus at present, it has been found that the piezoelectric properties of PVDF are ascribable mainly to the magnitude of C--F dipole moment and .beta.-crystals having spontaneous polarizability. As a result of further studies to increase the dipole moment and to stabilize the orientation of dipole as a mean of molecular design, it has been discovered that vinylidene cyanide copolymers having a C--CN dipole moment which has a value higher than that of the C--F dipole moment, when subjected to a polarization treatment, exhibit excellent piezoelectric performance (Japanese Patent Application (OPI) No. 69818/81 (the term "OPI" as used herein means a "published unexamined Japanese patent application").
It has now been found that if vinylidene cyanide copolymers are subjected to a heat treatment prior to or simultaneously with poling, polymeric electret elements which have a large longitudinal piezoelectric effect (Kt) (although they have a poor transverse piezoelectric effect (d.sub.31)) and are thermally stable over long periods of time, can be obtained. The present invention has been accomplished based on these findings.